Friend and former colleague Roland Bouwman has written an excellent update on the GROUP BY implementation in MySQL.
MySQL’s implementation of GROUP BY has historically been quirky. Sometimes that quirkiness has been useful, but often it causes grief as SQL authors can make mistakes that are executed but don’t produce the results they want (or expect).
SELECT cat, COUNT(val) as cnt, othercol FROM tbl GROUP BY cat
The ‘cat‘ column is in the GROUP BY clause, the COUNT(val) is an aggregate, but the ‘othercol‘ column is … well… neither. What used to effectively happen is that the server would pick one othercol value from within each group. As I